One man, in his 60s, suffered broken ribs and bites to his hands after a dolphin rammed him a few metres off Suishohama beach in the town of Mihama, Fukui prefecture, early on Sunday.

Another man, in his 40s, sustained arm bites in a separate incident on the popular beach the same morning.

Two more people were injured by the mammals later in the day.

Fukui has now recorded six such attacks this year, local police say.

Signs have been put up telling swimmers to avoid approaching or touching the mammals.

While dolphins are not usually aggressive to humans, hostility towards swimmers is not unheard of.

Scientists have suggested that wild bottlenose dolphins find swimming alongside humans incredibly stressful, finding evidence that it disrupts their behavioural routines.

In the Republic of Ireland, two women were injured in the space of ten days in 2013 by the same dolphin, including one who suffered from a broken rib.

A year later five swimmers had to be rescued off the Irish coast when a dolphin encircled them aggressively.

As well as hostility towards humans “ dolphins are known at times to behave extremely violently towards their fellow sea creatures.

In Cornwall, south-west England, a bottlenose dolphin was seen flipping a porpoise into the air as part of an aggressive attack. (BBC)